Abstract

Abstract
Zeolite structures are known to become distorted and strained on dehydration. In the present work, the results are given of thermochemical studies on the dehydration process in 17 natural heulandite samples covering the entire range of chemical compositions (heulandites 1, 2, 3). In each sample, a sudden change of the thermodynamic properties (heat capacity of heulandite, partial molar entropy of H2O, entropy production during dehydration) was detected at particular H2O contents. The direction of this change suggests a straining of the structure leading to a lowering of its entropy (i.e., ordering in the thermodynamic sense). The detected changes are interpreted as synergetic phase transitions. Correlations are found not only between the chemical compositions of the samples and the thermodynamic characteristics of the dehydration process, but also with the H2O content at which the phase transition begins. These correlations are different for heulandites 1 and 2 and clinoptilolites (heulandites 3), suggesting a difference in their dehydration mechanism; this makes it possible to discriminate between them more precisely according to their chemical compositions